Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_invocation_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import "dart:_js_helper";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 @Native("A")
 class A {
   var foo;
 }
 
 class B {
   var foo;
 }
 
 nativeId(x) native ;
 
 void setup() native """
 nativeId = function(x) { return x; }
 """;
 
 main() {
   setup();
   var b = new B();
   b.foo = (x) => x + 1;
   b = nativeId(b); // Inferrer doesn't know if A has been instantiated.
   // At this point b could be A or B. The call to "foo" thus needs to go through
-  // an interceptor. Tests that the interceptor doesn't screw with retrieving
-  // the field and invoking the closure.
+  // an interceptor. Tests that the interceptor works for retrieving the Dart
+  // class field and invoking the closure.
   Expect.equals(499, b.foo(498));
 }